Process for Trimerization

This invention relates to a process for the trimerization or cotrimerization of ethylene, propylene, and 1-butene.

Processes for the catalytic polymerization of the above-mentioned olefins are well known. One process of particular interest here is that described in U.S. Pat. No. 3,347,840, incorporated by reference herein. This patent raises the problem of the disadvantageous production of appreciable amounts of 1-hexene in the polymerization of ethylene, and suggests various compounds to counter or inhibit this unwanted trimerization, which detracts from the efficiency of the conversion to polyethylene.

It was apparent, however, that the catalyst utilized in the process, e.g., a chromium (III) salt, usually an alkanoate, in combination with a partially hydrolyzed aluminum alkyl, such as triisobutyl aluminum, was capable of producing 1-hexene, albeit in minor amounts, and much of the 1-hexene was incorporated into the polymer. This observation suggested that the ethylene polymerization catalyst could be used to trimerize ethylene in greater than the minor amounts mentioned provided that, instead of 1-hexene inhibitors, 1-hexene promoters could be found. In this vein, the above patent alludes to components, such as dimethoxyethane, which tend to increase the amount of ethylene converted to 1-hexene. There is no indication that the conversion would be at all efficient, however, and, in fact, following the teachings of the patent, it is not.

An object of this invention, therefore, is to provide a process for the trimerization or cotrimerization of ethylene, propylene, and 1-butene wherein a high conversion of olefin to trimer is achieved.

Other objects and advantages will become apparent hereinafter.

According to the present invention, a process for the trimerization of an olefin selected from the group consisting of ethylene, propylene, 1-butene, and mixtures thereof has been discovered comprising passing the olefin and carbon dioxide in contact with a catalyst comprising the reaction product of (i) a chromium compound, which will provide active catalytic species under trimerization conditions, and (ii) a hydrocarbyl aluminum hydrolyzed with about 0.7 to about 1.1 moles of water per mole of aluminum compound wherein the aluminum to chromium mole ratio is in the range of up to about 200 to one and the mole ratio of olefin to carbon dioxide is in the range of about 4 to about 3000 moles of olefin per mole of carbon dioxide.

The mole ratio of olefin to carbon dioxide is in the range of about 4 to about 3000 moles of olefin per mole of carbon dioxide and is preferably in the range of about 20 to about 1500 moles of olefin per mole of carbon dioxide. In terms of pressure, the partial pressure of, for example, ethylene would be in the range of about 100 to about 1500 psi and the partial pressure of carbon dioxide would be in the range of about 0.5 to about 25 psi.

Selectivity to trimer, in subject process when applied to the trimerization of ethylene, can be as high as eighty-five percent by weight or even higher based on the weight of ethylene reacted. With regard to ethylene, there is no significant production of branched trimer and almost all of the trimer is linear and terminal. Insofar as trimers and cotrimers of ethylene, propylene, and butene-1 are concerned, virtually little or no incorporation of trimer into the polyolefin by-product occurs. This can be verified by carbon-13 nuclear magnetic resonance spectroscopy and resin density measurements. Thus, there is produced what may be referred to as "free" trimer. Consequently, little or no separation of the desired product from other contaminating olefins is necessary. Essentially all of the trimer produced is recovered and it is of high quality with very little isomerization to the less desirable internal olefins. The major inefficiency is the formation of polyolefin homopolymer. Other low molecular weight oligomers are produced, e.g., in the production of 1-hexene, 1-butene and undefined octenes are produced, but these amount to less than about five percent by weight of the ethylene consumed.

The trimers produced by subject process are 1-hexene; nonenes; and dodecenes. Terminal 1-hexene is a desirable product for a number of applications, including copolymerization with ethylene and hydroformylation. It is advantageously used as a comonomer with ethylene since it imparts good tear strength to low density resins and has the required volatility and polymer incorporation properties. Terminal nonenes and dodecenes find a variety of applications similar to those of other long chain alpha-olefins such as in surfactants and lubricants.

The catalyst utilized in subject process is comprised of two components and, optionally and preferably, a third component:

1. The first component is a chromium compound, which will provide active catalytic species under trimerization conditions. It is understood that the chromium compound can be a mixture of chromium compounds. The chromium can be in the oxidation states of 0 to 6 with the oxidation states (II), (III), and (IV) being preferred. The non-metallic portion of the compound can be inorganic or organic. A typical formula for the chromium compound is CrX.sub.n wherein X is an inorganic or organic radical and n is an integer from 1 to 6. The inorganic radical can be, for example, a halide, a sulfate, or an oxide. Halides are not preferred, however. The organic radical can have 1 to 20 carbon atoms, preferably 1 to 10 carbon atoms, and can be selected from the group consisting of alkoxy, ester, or ketone radicals. The organic radicals can be straight chained or branched, cyclic or acyclic, aromatic or aliphatic, or can be made up of mixed aliphatic, aromatic, and/or cycloaliphatic groups. Preferred chromium compounds are as follows: chromium (III) tris(2-ethylhexanoate); chromium (II) bis(2-ethylhexanoate); and chromium (IV) tetra-tertiary-butoxide. Other suitable chromium compounds are chromous bromide; chromic bromide; chromous chloride, chromic chloride; chromous fluoride; chromic fluoride; chromium (II) acetate; chromium (III) oxy-2-ethylhexanoate; chromium (III) dichloroethylhexanoate; chromium (III) acetylacetonate; chromium (III) acetate; chromium (II) butyrate; chromium (III) butyrate; chromium (II) neopentanoate; chromium (III) neopentanoate; chromium (II) laurate; chromium (III) laurate; chromium (II) stearate; chromium (III) stearate; chromium (II) oxalate; and chromium (III) oxalate. In a homogeneous process, the chromium compound must be soluble in the trimerization medium under trimerization conditions.

2. The second component is a hydrocarbyl aluminum hydrolyzed with about 0.7 to about 1.1 moles of water per mole of aluminum compound. The hydrocarbyl aluminum can be represented by the formula R.sub.3 Al wherein each R is an alkyl, cycloalkyl, aryl, or hydride radical; at least one R is a hydrocarbyl radical; two or three R radicals can be joined in a cyclic radical forming a heterocyclic structure; each R can be alike or different; and each R, which is a hydrocarbyl radical, has 1 to 20 carbon atoms, and preferably 1 to 10 carbon atoms. Further, each alkyl radical can be straight or branched chain and such hydrocarbyl radical can be a mixed radical, i.e., the radical can contain alkyl, aryl, and/or cycloalkyl groups. Examples of suitable radicals are: methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, hexyl, 2-methylpentyl, heptyl, octyl, isooctyl, 2-ethylhexyl, 5,5-dimethylhexyl, nonyl, decyl, isodecyl, undecyl, dodecyl, phenyl, phenethyl, methoxyphenyl, benzyl, tolyl, xylyl, naphthyl, methylnaphthyl, cycohexyl, cycloheptyl, and cyclooctyl. It is understood that the donor ligand described below can be incorporated into the hydrocarbyl aluminum structure such that the hydrocarbyl radical includes isonitriles, amines, and ethers.

The hydrolyzed hydrocarbyl aluminum is also known as an aluminoxane, which is thought to be a complex mixture of unknown stoichiometry.

The synthesis can be described by the following equation:

A typical hydrolysis can be carried out at atmospheric pressure at a temperature in the range of about 0.degree. C. to about 100.degree. C., and preferably at a temperature in the range of about 10.degree. C. to about 65.degree. C. Water is added to a solution of, for example, trialkyl aluminum in an anhydrous, inert, organic solvent. The concentration varies from about 5 percent by weight aluminum compound to about 75 percent by weight based on the total weiqht of the solution. The water is preferably added slowly, but in a single batch, with vigorous stirring and cooling. The reaction is considered complete when effervescence ceases.

Another method for preparation of the aluminoxane is accomplished by using the waters of hydration of metal salts, e.q., by adding about 0.1 to about 0.16 mole of solid magnesium sulfate heptahydrate to a ten percent solution of triisobutyl aluminum in heptane. The mixture is stirred vigorously until effervescence ceases, usually overnight.

The solutions are stored under an inert gas such as argon.

Examples of suitable solvents are heptane, 1-hexene, hexane, pentane, isooctane, purified kerosene, cyclopentane, cyclohexane, methylcyclopentane, and dimethylcyclopentane. The use of 1-hexene is found to be advantageous in some cases. Benzene, toluene, and xylene can be used, but are not preferred. It will be understood that these solvents can be used both in the preparation of the chromium salt and the aluminoxane, and in the trimerization process itself, and that minor products of trimerization such as octenes will become part of the solvent on recycling.

Examples of hydrocarbyl aluminum compounds are as follows: triisobutylaluminum, trihexyl aluminum, di-isobutyl aluminum hydride, dihexyl aluminum hydride, isobutyl aluminum dihydride, hexyl aluminum dihydride, di-isobutylhexyl aluminum, isobutyl dihexylaluminum, trimethylaluminum, triethylaluminum, tripropylaluminum, triisopropylaluminum, tri-n-butylaluminum, trioctylaluminum, tridecylaluminum, tridodecylaluminum, tribenzylaluminum, triphenylaluminum, trinaphthylaluminum, and tritolylaluminum. The preferred hydrocarbyl aluminums are triisobutylaluminum, trihexyl aluminum, di-isobutyl aluminum hydride, and dihexyl aluminum hydride.

3. The third component belongs to a group of ligands, which can be characterized as electron donors. The ligands are compounds selected from the group consisting of hydrocarbyl isonitriles, amines, and ethers. Since a ligand can be a polymer, theoretically the number of carbons contained in its structure can be considered to be essentially unlimited. It is preferred, however, that each ligand have 1 to 20 carbon atoms. The hydrocarbyl portion of the ligand can be straight or branched chain alkyl, cycloalkyl, or aromatic, or a mixture of the foregoing. The ligands can be represented by a formula containing central donor atoms or groups, which are limited to the elements carbon, oxygen, and nitrogen. The formula follows:

wherein

each R and R' is a straight or branched chain algyl, cycloalkyl, or aromatic radical, or a mixture of the components which make up the foregoing radicals;

R and R' can be alike or different;

each E, E', and E" is a donor atom selected from the group consisting of oxygen, nitrogen, and an isonitrile carbon;

E, E', and E" can be alike or different;

m, m', m", n, and n' are integers from 0 to 3 selected to satisfy the valence requirements of the donor atoms;

any two R and/or R' radicals can be joined to form a single cyclic radical;

R is bonded to one of E, E', or E";

R' is bonded to two of E, E', or E"; and

a and b are any integers.

Examples of the application of the formula follows:

(i) for the monodentate ligand, diethyl ether:,

R=ethyl, E=oxygen, m=2, a=0, and b=0; and

(ii) for CH.sub.3 O(CH.sub.2 CH.sub.2 O).sub.3 CH.sub.3 :

R=CH.sub.3 ; R'=CH.sub.2 CH.sub.2 ; each E, E', and E" is an oxygen atom; m=1; m'=0; m"=1; n=1;n'=1; a=2; b=1.

Ligands, which exhibit a high selectivity for free 1-hexene are as follows:

CH.sub.3 O(CH.sub.2 CH.sub.2 O)CH.sub.3 ; also known as monoglyme

CH.sub.3 O(CH.sub.2 CH.sub.2 O).sub.2 CH.sub.3 ; also known as diglyme

CH.sub.3 O(CH.sub.2 CH.sub.2 O).sub.3 CH.sub.3, also known as triglyme

tert-butyl isonitrile

tetrahydrofuran

C.sub.6 H.sub.4 (OCH.sub.3).sub.2, also known as veratrole

Other useful ligands are tetramethylethylenediamine, 1,2-diethoxyethane, CH.sub.3 O(CH.sub.2 CH.sub.2 O).sub.4 CH.sub.3 also known as tetraglyme, 1,2 dimethoxypropane, 2,3 dimethoxybutane, o-diethoxybenzene, methylisonitrile, ethylisonitrile, phenylisonitrile, para-tolylisonitrile and tetraethylethylenediamine.

As noted, the aluminum to chromium mole ratio is in the range of up to about 200 to one. The preferred aluminum to chromium mole ratio is in the range of about one mole of aluminum to one mole of chromium to about 100 moles of aluminum to one mole of chromium with the lower ratios being preferred. Thus, the most preferred aluminum to chromium mole ratio is in the range of about one to one to about 50 to one. The ligand to chromium mole ratio is in the range of up to about 100 to one, and is preferably in the range of about one to one to about 100 to one. Within these ranges for the ligand to chromium mole ratio, each ligand has its own preferred ranqe, which is determined experimentally. The optimum range for most ligands falls within the one to one to 40 to one range. Insofar as selectivity to free trimer is concerned, preferred ligands are selective in the range of 40 percent to 85 percent by weight or above based on the weight of olefin reacted. There is no preferred order for mixing the two or three components.

The catalyst is effective in the trimerization process at temperatures in the range of 20.degree. C. to about 200.degree. C. Preferable temperatures are in the range of about 70.degree. C. to about 140.degree. C. Effective pressures are in the range of atmospheric to about 1500 psig or higher. Typical concentrations of chromium are in the range of about 10.sub.-3 millimoles per liter to about 10 millimoles per liter with other catalyst components scaled accordingly. The volume (measured in liters) referred to here includes the catalyst materials together with solvent, which will be utilized in the trimerization process.

The three component catalyst is discussed in U.S. Pat. No. 4,668,838 to be issued on May 26, 1987. This patent is incorporated by reference herein.

The catalyst described in this specification is most easily employed in a homogeneous process such as that set out in the examples, but can be modified to operate as, for example, a heterogeneous silica-supported catalyst. Techniques such as those described in "Studies in Surface Science and Catalysis, Volume 8, Catalysis by Supported Complexes", edited by Yermakov, Elsevier Publishing Company, 1981, could be employed to achieve this objective.

The invention is illustrated by the following examples.

All operations involving the use of catalyst components are conducted under an inert argon atmosphere. Heptane solvent is first stirred over concentrated sulfuric acid for 2 days to remove water, aromatics, and other unsaturates, then stored over and distilled under argon from calcium hydride or a sodium/potassium alloy before use. Other additives are purified in a conventional manner, e.g., monoglyme is distilled under argon from sodium metal and stored over molecular sieves in the dark. Ethylene is polymer grade. It is purified to polymerization process specifications and further dried gy passage through molecular sieves.

Chromium (III) tris(2-ethylhexanoate) is typically prepared from anhydrous chromium (III) chloride and 2-ethylhexanoic acid as follows: About 110 cubic centimeters (cc.) of 2-ethylhexanoic acid is heated to 130.degree. C. and sparged with argon for about 2 hours to drive off any contaminating water. After cooling to 80.degree. C., 11.2 grams of anhydrous chromium (III) chloride is added over a period of 20 minutes. The temperature of the mixture is raised slowly to 230.degree. C. over a period of 6.5 hours before being cooled. The mixture is then stripped under a vacuum of about one millimeter of mercury at 160.degree. C. to drive off unreacted acid and other volatiles. This gives a glassy green solid. The product is extracted from unreacted chromium (III) chloride and other insoluble material with three 100 cc. portions of diethyl ether, filtered, and stripped under vacuum.

Hexaisobutyltetraalumoxane is used as received from the manufacturer.

The examples are carried out in an essentially air-free one liter stirred autoclave at 85.+-.5.degree. C. and 250 psig total pressure. 0.2 millimole of chromium (III) tris (2-ethylhexanoate), 6.0 millimole of hexaisobutyltetraaluminoxane, and, where used, 1,2-dimethoxyethane are mixed in hexane outside of the reactor and then transferred to the reactor, which contains 500 milliliters of nitroqen purged hexane as a diluent. Carbon dioxide, where used, is added and the reactor is heated to 80.degree. C. and pressured to 250 psig with ethylene. Ethylene is fed on demand to maintain pressure and temperature is controlled by adjusting the temperature of water in a jacket surrounding the reactor. At the end of two hours, the reactor is cooled, vented, and opened. The diluent is analyzed for 1-hexene. Polymer yield is determined by evaporating the diluent and weighing the residue.

1-Hexene Efficiency is given in percent and is determined as follows: ##EQU1##

The variables and results are set forth in the following Table.

The product obtained contains 1-hexene and polyethylene in a combined total of at least 98 percent by weight based on the weight of ethylene consumed. The balance of the product is represented by undefined octenes, 1-butene, and cis and trans 2-butene.